NG_NAS_TypeDefs.ttcn: Fix dec of NG_SECURITY_MODE_COMMAND

The iei field in nasSecurityAlgorithms is filled by the decoder, which should be
decoded as "omit".
type record NAS_KeySetIdentifier already has a variant "FORCEOMIT(iei)"
in its definition, but according to "Programmers' Technical Reference
Guide for the TITAN TTCN-3 Toolset", FORCEOMIT "Forces the lower-level
optional field(s) specified by the parameters to always be
omitted. It does not affect the own fields of the defining record/set."

Applying the variant in the parent-level record fixes the decoding
issue.